text:
How is combustion flame maintained in the combustion chamber after igniters are switched off?
During start phase of a jet engine, igniters in the combustion chamber create sparks to initiate combustion.  As far I know, when the engine reaches self sustaining speed, igniters get turned off (this might be different (takes longer) in an Airliner due to FAA regulations during take off).
My question is how do flames in the combustion chamber (or canisters) get sustained (like a candle) without igniters support?

Possible duplicate of Why do gases in the combustion chamber only flow one direction to the gas turbine in a jet engine?

Another answer to add to caseys:

Flow speed near the fuel injectors is very slow, on the order of 30 m/s. This is caused by a widening of the flow path between compressor exit and combustor called a diffusor or diffuser. See here for more.
Radiation from the burning fuel-air mixture in the middle part of the combustor heats up the not-yet-burning mixture near the fuel injectors, which helps the fuel droplets to evaporate (this is a necessary step before combustion can start) and causes the mixture to be heated above its autoignition temperature. Hence, this radiated heat will provide the fuel-air mixture with the needed activation energy for combustion.

Some combustors have small notches in their wall which create local turbulence and help to keep some of the burning fuel-air-mixture back, which helps to start ignition in the following flow. These are called flame holders. In afterburners, where flow speed is much higher, they can be rings of a V-shaped cross section, with the pointed end of the V pointing forward.
If all of the burning gas is washed out from the combustion chamber without igniting the following gas, the ignition process will stop. This is called a flame-out. The heat from the walls of the combustion chamber will not be enough to re-ignite the fuel-air mixture, because they are cooled by the airflow from the compressor and will rapidly cool down once the flame is blown out.
Note that this effect is used to extinguish burning oil or gas wells with dynamite.

How does a candles flame sustain itself after you have lit it?  It stays lit by continually burning fuel.  The jet engine works the same way.  Once the fire is lit (by the ignitors) it is constantly burning.  Air is constantly being supplied by the compressor and fuel is constantly being supplied by pumps.
This is fundamentally different to a traditional combustion engine you might be trying to reconcile this idea against.  In a 4-stroke internal combustion engine, for example, you have intake, compression, power and exhaust happening separately and you need spark plugs (or glow plugs) to ignite the fuel each cycle.  In a jet engine, however, instead of 4 independent cycles you have a constant flow of air through the engine.  Air comes in the front, is compressed by the N2 stage and fed continually into the hot section where the fire is constantly burning and then exhausts through the back of the engine.
Look at it like a propane barbecue grill or a gas stovetop -- you only need the ignitor to start the fire. Once the fire is burning it stays that way until you turn off the flow of gas.

I think the asker is wondering why the 500mph airflow through the jet engine doesn't simply blow out the flame -- if you blow hard enough on a candle or a BBQ grill, you'll blow out the flame... why doesn't the same happen in a jet engine? I suspect that the answer involves the compression stage where the airflow is slowed as it's compressed before fuel is injected and the mixture is ignited in the combustion chamber which constrains the burning gases to allow combustion before exiting through the turbine?

@Johnny Thanks for elaboration.  Yes, I exactly wonder why high speed air from compressor does not blow out the flame.  I just wonder if the temperature of the compressed air has a role for ignition? Or compressed air in 30000 feet has enough temperature to ignite the fuel?

I have the same question as the OP, and yeah, this doesn't answer the question for the same reason as in the above comments.

Minor point - glow plugs are only used to help start a diesel engine (when it is below operating temperature).  After that it is self-sustaining, albeit for different reasons than for a jet engine.  A diesel/air mixture ignites spontaneously simply by result of the heat of compression.

Further reading here

Also note that relevant flame front advance velocities apply to the combustion chamber.  And these are generally exceeded by air velocity, so flame is held usually by swirl or bluff body flame holder.  This zone is always receiving a rich fuel mixture and is at high temperature.  This flow is diluted by annual intake into the chamber where it continues combustion having left the flame holder.

To add to casey's post, the flame holder keeps the flame from getting blown out. Its a part of the combustion chamber that allows the high pressure air from the final compression stage to swirl at a low speed to keep the flame front contained.

As far as I know the flame holder is placed right after gas turbine, not in the combustion chamber.

@albin: The flame holders behind the turbine are for the afterburner and much bigger than the ones in the combustion chamber.


text:
Are those devices allowed on a flight which create their own WiFi signals?
I am going from Canada to Mexico but I don't want to be dependent on in-flight entertainment. Thus I got seagate wireless plus so I can stream to my iPad. I couldn't find any policies I can use this device during flight or not. At very least I am trying to determine if it is worth asking the flight company?

There is only one possible. correct answer.  Ask the airline.

No, the rules are about any transmitter or receiver, regardless of whether it comes under the specific category of phone/tablet/laptop etc
If anything your device is worse as it transmits at a higher power and range than a receiver.

I see I was just unsure with introduction of Wifi on some flights so I thought they might have gotten a little less strict on this issue.

@Pipsydoodles The airline can install certified WiFi devices on their aircraft. That doesn't mean passengers can use any random wireless transmitting device onboard.

The access points on the airplane might be certified, but the computers in use are not. The computers transmit as well as receive (that’s how you send email, make requests etc). There’s no huge difference in a computer that mostly receives and a “seagate wireless plus” that mostly transmits. Therefore, no reason why the seagate device would be “worse”. Also, the 802.11a/b/g standard specifies a maximum transmission power which will be the same for computers and for the seagate device.

Do you have any reference for this?

802.11 specifies: "The maximum allowable output power is measured in accordance with practices specified by the appropriate regulatory bodies." These bodies do not discriminate between access points and clients; the 2,4 GHz band is generally free for anyone and anything to use (up to a maximum signal strength). Though not optimal, in practice, nearly all wifi adapters are configured to transmit with maximum allowable power. If anything, given its uses, the Seagate device is more likely to use a conservative power level.

If you couldn't have WiFi transmitters on board then you couldn't use WiFi at all. To connect to WiFi requires a transmitter. All laptops, tablets, phones connected to WiFi are transmitting on 2.4GHz.

Airlines provide WiFi onboard and invite you to connect to it (for a fee). To connect to WiFi you must have the ability to transmit (internet requires two way communications). If your device is conforms to FCC power limits on the 2.4GHz bands (or whatever your particular jurisdictions regulatory body is) then your device is no different than any other WiFi device on the plane. They are all transmitters using the same maximum power.
If your airplane has WiFi onboard and let's you connect computers to it then there is no technical problem with using your device.
Of course, if a crew member tells you to turn it off, remember you must comply.

The rules are FCC rules, not FAA rules.
The rules about cell phone signals being off during flight have to do with the technical challenges of mobile phone carriers and their cell sites that have to hand off location info about every device in the "cell." At 600 mph this gets challenging.
A correctly working phone is not going to cause a problem for the airplane's communications or avionics systems, but with literally a million passengers in the air at any given time, the cell towers are going to go crazy. They were designed for driving speeds and slower.
Turn everything off at all times below 10,000 feet. Especially takeoff and landing.
And if any member of the flight crew tell you to turn something off, regardless of their misunderstanding about the rules, laws, safety guidelines, etc. - just follow their orders.

The intention behind is to isolate the electronic devices of an airplane from electromagnetic radiation.
Modern airplanes are pretty well protected at the moment and they don't have any issue of isolation.
However, one thing that is key for authorities is flight safety. So it is better to have stronger restrictions that making any mistake.
So, in order to be able to use wifi on the airplane, you need to have, first the airplane modern enough and the airline shall be authorized by the authorities.
Finally, in any case those radiation devices should be turned off (or in airplane mode) when flying in take-off or landing stages (so the wifi of the airplane does).
My recommendation: ask the airline directly and get information about the airplane you are flying in.

The airlines will be primed to say No. But you will hardly be the only one on board with an iPad so it's unlikely anyone will notice. Turn it off if the plane suddenly pitches nose-down at the beginning of your movie.
However, I question the need. I have a 32Gb iPad and there's absolutely no way I can watch a full load of video on my entire vacation, never mind the flight at each end.

Are you seriously suggesting that someone violate aviation regulations since "it's unlikely anyone will notice"?

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
How can flight attendants respond to an immediate medical emergency?
When an airplane is about to takeoff (after leaving the gate), how should the flight attendants respond to an emergency with a passenger which requires medical immediate assistance?

If the pilot knows before committing to take-off and reaching the point of no return then he will abort the takeoff and ask the tower to send an ambulance and stairs.
If he is committed to take off then he will get into the air and then tell the tower of the emergency; circle around (possibly dump fuel depending on the severity) and land again to get the passenger off board on the ambulance waiting near the runway.
Whether the plane can then top up the tanks and take off again will depend on whether the landing was overweight or not. If it was then the plane needs to be inspected before it can be put back into use and the other passengers will need to leave the plane and wait until the next plane can ferry them on.
The cabin crew will apply first aid until the ambulance crew can take the sick passenger.

It might be worth noting that being airsick or having a panic attack due to fear of flying might not always constitute an emergency requiring termination of the flight.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What would happen if the sole pilot dies while flying an aircraft?
An aircraft is flying in cruise flight on a long distance flight. The autopilot is turned off. The only occupant of the aircraft is the pilot.
Suddenly that pilot dies, what will happen?

This question is very broad - can you give a more specific context to the question? Or at least remove some ambiguity? eg in this scenario do we assume that the pilot dies without moving the controls or holding them in position? Is the aircraft assumed to be trimmed in level flight?

@JonStory, Yes, aircraft is flying in right direction and pilot dies sudden.

it will crash eventually?

It has happened before that the single pilot lost consciousness while flying. What then happened is that the plane just went on until out of fuel after which it will glide down and skid to a stop (pilot survived). This is assuming there is no obstacle in the way.
Another example is the cornfield bomber, a fighter pilot eject and the plane still under power just kept going in a straight line and "landed" in a field.
A more deadly example is the helios flight 522. A mistake in the pressure regulation setting let the plane become unpressurized while climbing to cruise altitude. Both pilots lost consciousness and the plane's autopilot followed its programmed path following the waypoints and ending up in a holding patter until it ran out of fuel and crashed into the ground.
Survival depends on luck with obstacles, whether the plane is properly trimmed and which order the engines flame out in, asymmetric flame-out may result in a spin.

Note that the first example in this answer assumes that the aircraft is properly trimmed for level (ish) flight, and that it is able to slowly descend, or trimmed such that it will gently descend when losing power... most aircraft will not be nicely trimmed, and without auto-pilot will usually crash.

@JonStory: Depends on autopilot type. Many (in large aircraft probably all) adjust trim now and then so the plane is not too badly out of trim when the autopilot is disconnected.

There was that jet with the golfer in it, they thought decompression or something, took out everyone in it, it flew for hours North.

@CGCampbell: I think you mean the Payne Stewart crash.

Assuming no autopilot the plane will likely crash very soon as even if the airframe was trimmed at the time of pilot's death as there will be no mechanism correcting trimming on outer forces changes (turbulence, pressure change, etc.) Imagine a driver dying in a car - if the road is straight and wide, the car may go on for a while but as soon as there is just a small change in roads direction, uneven surface or even stronger wind, the car will crash quickly. Some planes may have features not directly tied to autopilot that correct piloting mistakes to some extent which may prolong the planes survival.
Also very important is what happens with the pilot's body. As long as the body doesn't interfere with plane controls, the plane will likely go forward longer. If, on the other hand, the pilot's body say falls on the controls, it may even disengage the autopilot if present and force the plane to make a move that will eventually go over its physical limits and results in spiral, stall, dive or disintegration in the air.

This is not correct. Most aircraft will fly (roughly) straight and level with no control inputs if they're trimmed for cruise flight. Of course, when the fuel runs out, it'll start descending. And this is without any computer assistance of any sort. This is due entirely to the aerodynamic stability designed into the airframe itself. There will be some oscillations, especially when the attitude is perturbed by turbulence, but they'll self-correct in most circumstances.

Also, the reason a driver will crash is that there are lots of things to crash into. If a driver dies in a wide-open area with almost nothing to hit, the car won't crash for a while either.

Aircraft trimming and leveling is what autopilot is supposed to do (if present), right? My understaning is that aircraft must be trimmed at any aerodynamic change of the environment, hence any pressure changes, turbulences or other fenomena will most likely result in unleveled airframe. That being said, I'm not a pilot so correct me if I'm wrong, please.
The driver analogy was just an analogy, although there are many forces that can cause a car to crash even on what seems to be a straight road.

@reirab Aircraft typically have pitch stability although pitch oscillations may occur. Roll stability is another matter. At best, an aircraft might end up flying in a wide, wobbly circle, but could just as easily enter a spiral dive.


text:
Is a slip an intentional consequence of a cross-control?
In flight training, you're always told not to cross-control, (for example, rolling right aileron, and stepping on the left rudder), but it seems to me that is exactly what a slip is doing.
Is it that a slip is intentional, and a cross-controlled situation is typically unintentional?

You are right: A slip is a cross-controlled (and uncoordinated) situation. It's exactly what your instructor was teaching you to avoid during your early lessons.
As far as nomenclature goes: all slips are cross-controlled maneuvers, but not all cross-controlled maneuvers are slips: On climb-out in most piston singles you will be stomping on the right rudder to offset the plane's left-turning tendencies, and you will probably notice that you're rolling in a tiny bit of left aileron (which offsets the banking tendency introduced by the rudder) - your controls may be very slightly crossed, but the result is coordinated flight (a centered ball).

The real difference here between an "unintentionally cross-controlled situation" and an "intentional slip" is the intentional bit: When you are knowingly slipping the aircraft - for example to bleed off altitude on a high final without increasing your airspeed - you're aware that you're flying in an uncoordinated and aerodynamically unstable condition. You're aware of the risks, and are hopefully monitoring the aircraft carefully to avoid a stall/spin.
For a little more insight, faaflighttest.us has a reprint/copy of an Aviation Safety article from April 2006 which is worth a read, and possibly an hour in the air practicing some of what they discuss with your instructor so you can get the "feel".

One other point is that a slip is done with sufficient airspeed that it won't create a problem, other than the (desirable) loss of performance that it causes.  Where being cross controlled really becomes an issue is when you are getting close to your stall speed!

Its also worth mentioning that in some aicraft the preferred crosswind approach is a cross-controlled maneuver called wing down, top rudder. For example, if the wind is blowing right to left, tye right wing is dropped to counter the drift, and then left rudder is applied to maintain RoD and centerline.

@Lnafziger most airspeed indicators don't provide correct indication in a fully cross-controlled slip, so don't look at the ASI (which will be too loow). Instead, the important thing is to keep the nose well below the horizon.

"you're always told not to cross-control"

This should not be what you're taught. You should fly in coordinated flight, and that can often mean being cross-controlled.
Two very common examples of coordinated, cross-controlled flight are left hand turns on take off and steep turns.
Because of p-factor, on departure, you will have some amount of right pedal in, but to turn left (particularly in a nosewheel airplane), you need left aileron while you still have right pedal in, and its not unusual for full-power climbing left turns to have left aileron and right pedal in coordinated flight. You'll see this more in a high-performance airplane.
The second example is steep turns. After you roll into a steep turn to the left, say, the over-banking tendency will require right aileron with left pedal to keep the plane from increasing bank angle, in coordinated flight. The over-banking tendency is less pronounced below 45 degrees of bank in low aspect-ratio aircraft, like your typical Cessna.
So the rule is not "don't cross-control," but rather "fly coordinated"
That being said, the easiest way to get a plane into a spin is full cross control, and pull back on the stick. That's probably why you're taught never to do it.

Great answer! I will change the way I talk about this. "Coordinated flight" is a great keyword and the better way to talk about it.

When you are doing a forward slip for example to bleed off altitude without increasing airspeed, you are purposefully uncoordinating the plane.  That is the entire purpose of a forward slip in that particular situation; the reason it is useful is because you are intentionally increasing the surface area of the plane that has to push it's way through the air/relative wind while still maintaining the ability to descend in a controlled manner.  This is why I don't prefer rules that are very general; Your rules at the time should be based on your desired outcome.  That isn't going to be the same in every situation... Although I'm speaking very generally here :p  Rather than have a set of one size fits all rules, I think it is better to keep keen situational awareness and have a set of rules for different circumstances.   very rarely do I enjoy a rule that says "never do this" or "always do this" save for "always be prepared and plan accordingly when possible".


text:
What are design or functional differences between sticks and yokes?
I've been flying Cessna 172 & 182's my whole flying life up to this point.  I know that Cirruses (Cirri?) have a "side yoke" and Diamonds, Citabrias and others have control sticks coming up through the seat.
Is this a design difference or are there actual functional differences between airplanes that have control yokes (like Cessna 182's) or sticks (like Diamonds) or side yokes (like Cirrus)?
For example, I realize that with the side yoke offers more legroom perhaps, but are there reasons beyond that - like pull/push force required?

There is some link between the type of control and the design maneuverability of the aircraft.
Consider a yoke.  Imagine twisting it to bank.  You are moving it with a twisting motion of one or both arms and in general, aircraft fitted with such yokes are capable of low to moderate roll rates.  Big displacements are uncommon but small, rapid, alternating changes are common.  Watch a wide body PF making control movements during a landing with any kind of xwind or turbulence.  They are less sensitive and give more visual feedback as to the control positions.
Now consider a stick, central to the pilot flying.  This is moved with a movement of the arm, generally only one, and can be rapidly displaced easily.  This is normally associated with aircraft with high rates of roll, especially in an aircraft designed to rapidly null out or reverse the roll.   It is also physically easier to move a stick than a yoke under high-G as the arm movement gives a longer lever.
A great example of this was the Vulcan which, despite its size, enormous wing area and large fin, was so rollable that it could out-maneuver the interceptors of its day.  It has a control column rather than a yoke and the PF would throw it around at altitude as if it were a fighter.  This would not be easy to do with a yoke.
I'm a bit long in the tooth and haven't flown fixed wing for many years.  I am not familiar with the rationale behind side sticks which are fitted to both low and high performance aircraft.  I am guessing they are fitted more for reasons of ergonomics than for performance.

Ergonomics (and cockpit layout) is a huge reason to use sidesticks. They free up the space in front of the pilot a lot, and make the usable space in the instrument panel much larger.

Great answer! Thanks! I never thought about being able to visually distinguish control movements as a benefit.

@egid: should that not be an answer?

Side-sticks, being smaller, are less precise. So they are only used in aircraft with augmented controls, i.e. the fine adjustments to stabilize the flight path are done by computer.

This answers a question I've thought about a lot. Thanks! Interestingly, if you look at fighter aircraft (particularly from WW2) I believe they all had sticks EXCEPT the twin engined P38. I always wondered about that one.

@JanHudec - the HP-18 sailplane had a sidestick controller.  No augmentation of flight controls there!

I'm posting only to confirm a detail mentioned in Simon's excellent answer, since I've flown all three of the aircraft mentioned in the question (Cirrus, Cessna, Diamond), which he hasn't.
The bit about side yokes, which he mentions he isn't familiar with, is completely correct.  To reiterate:
Control type ordered by ease of control input

Center stick (easiest)
Side stick / side yoke
Standard (center) yoke (least easy)

Similar to the usual request for "define efficient", you should define what "easy" means in this context.

I think you will not find side yokes (side sticks) on older aircraft that have purely mechanical connections to the control surfaces.  Sometimes they require both pilot and copilot to shove on the controls to get the desired result.  Look at how robust the controls are on a DC-3 for example.
Other than the force required, it is merely a design issue.  Potato, Potatoe.

The newer airplanes that he mentions (Cirrus) are also "purely mechanical connections to the control surfaces", so I don't see what the age of the airplane has to do with it.

I don't see what force required has got to do with it. With powered flying controls and FBW, very large control forces can be generated and countered with little force on the yoke or stick.  Often, these forces have to be clamped and damped to prevent the pilot from tearing bits of the aircraft - think about fin losses following rapid rudder reversals.

The Cessna 400 has sidesticks, and uses fully mechanical pushrod linkages.

Modern airliners use sidesticks..

@Dan: depends on what you call "modern", there are quite a few aircraft out there certified recently with yokes.

@Qantas94Heavy I didn't mean all airliners, but I meant to further disprove the given answer

@Dan: the answer states that older aircraft don't use sidesticks, which your comment doesn't disprove... ;)

@Dan AFAIK, airliners with sidesticks are all fly-by-wire; at the very least, zero modern airliners have purely mechanical connections to control surfaces (they all use hydraulics as the primary control mechanism).

@Simon "purely mechanical connections to control surfaces" -- FBW is not that, nor are hydraulics.

@SkipMiller - the HP-18 sailplane had a sidestick controller.  No augmentation of flight controls there!  Purely mechanical conections.  Also, it certainly qualifies as "older".


text:
Is it possible to hand start modern planes?
I know that older planes must be hand started but are newer GA planes even able to be hand started by spinning the prop?  If so, how do you do it safely without cutting your hands off or being pulled in by the propeller?

Why should it not work? If in doubt, have a look on youtube. Plenty of hand start videos there.

How do you do it safely? The same way they did it safely 100 years ago - work quickly, work smart, and stay the heck out of the prop-disc area once you've given it a tug!

Related

Specify what you mean by 'modern' and 'older'.  It's possible to start a mid-60s Cherokee by hand-propping - I've done it several times. It's even possible to do it by yourself, when you're the only person at a remote strip, and discover your starter doesn't work any more.

You sure can!
In reality the engines in modern prop planes are not all that different (in some cases even identical) to the engines of the hand crank days. The reason many older planes were hand crank was simply to save the weight toll of a starter motor and battery to drive the system. Some older aircraft have no active electronics (the piper cub comes to mind). Remember the engines magnetos drive the spark for the ignition system unlike in a car where the spark is driven by the electronic system. As long as proper precaution is taken its more than possible. The FAA even explicitly discusses it in their airplane flying handbook (p 2-13)

A spinning propeller can be lethal should it strike someone.
  Historically, when aircraft lacked electrical systems, it was
  necessary to “hand prop” an aircraft for starting. Hand propping an
  aircraft is a hazardous procedure when done perfectly. The
  consequences of not mitigating the hazards associated with hand
  propping can lead to serious injury, fatalities, and runaway
  airplanes. All alternatives must be considered prior to hand propping
  an aircraft and, when a decision is made to do so, the procedure must
  be carried out only by competent persons who have been trained to
  accomplish the procedure, understand how to mitigate the hazards, and
  take all the necessary precautions.

They also provide a pretty clear checklist on how to do it.

The procedure and commands for hand propping are:
• Person out front
  says, “GAS ON, SWITCH OFF, THROTTLE CLOSED, BRAKES SET.” 2-14
• Pilot seat occupant, after making sure the fuel is ON, mixture is RICH,
  magneto switch is OFF, throttle is CLOSED, and brakes are SET, says,
  “GAS ON, SWITCH OFF, THROTTLE CLOSED, BRAKES SET.”
• Person out front, after pulling the propeller through to prime the engine says, “BRAKES
  AND CONTACT.”
• Pilot seat occupant checks the brakes SET and turns
  the magnetos switch ON, then says, “BRAKES AND CONTACT.”

As far as i know, you can not hand start a turbo prop (which some GA planes have) due to the higher RPM needed for start.

Note that the advice above and in fact most advice about hand-propping applies to direct drive engines where the propeller is bolted right to the crankshaft. Geared engines (Rotax probably being the ones most folks are familiar with) can be hand-propped, but they are generally not well suited to it. Also six cylinder engines or ones with 3-bladed propellers (or more) are generally considered to be a bit more of a chore to hand-prop.

Turbines can't be hand-propped not just due to higher RPM needed, but even more so due to continuous RPM needed. With piston engine, you just need is to push one cylinder through the compression, so quarter to half a turn is all you need, while a turbine needs to spin for a couple of seconds to build up the pressure. Also, in most turbines, the prop is actually not connected to the compressor at all.

If so, how do you do it safely without cutting your hands off or being pulled in by the propeller?

The short answer is "very carefully."
A longer answer, and the way my father taught me to do it (but not necessarily how others might do it) when I was in my early teens was as follows:

Stand close enough to the propeller disk so that you're not leaning forward. The closer the better, but remember that if you're propping a taildragger the bottom of the disk is closer to you than the top. Beware if wearing clothing that has lot of loose fabric that might contact the propeller. The two times that my father was slightly injured was due to contact between the propeller and lose fabric when propping taildraggers.
Rotate the propeller with the switch off until the blade you'll be propping is at about the 10 o'clock position and just before a compression cycle, then switch on.
Place the palms of both hands on the blade about 85% of the way out from the center. The tips of your fingers should be at the trailing edge of the blade, but be careful not to wrap your fingers around that edge.
The next part was tricky but was necessary because both my father and I were small and light. It worked best for us if we swung our left leg (it may have been the right leg but I really can't remember since this was over 60 years ago) up into the propeller disk and then smoothly, briskly, and aggressively bring the propeller blade down while swinging the leg down and out of the way, continuing into a step away from the propeller.

Radial engines, because of their lower compression, were easier to prop than inline engines. However, I wasn't tall enough when young to handle their height.
The aircraft I hand propped the most as an adult was an 85 horse Continental on a J-3 floatplane. You started it on one mag, and that mag had a spark delay for starting that didn't fire until the bottom of compression rather than the top. You steadied yourself on the front of the right float with your left hand grasping the fuel tank filler cap and nozzle just in from of the windshield and propped with your right hand. Because you were propping from behind, you had to use your fingers over the blade trailing edge, but the spark delay obviated the danger of backfire.

Very good points. I've hand-propped Lycoming 180s many times, and this advice fits perfectly. I had no one to teach me in that much detail, but what you describe is the procedure that came natural to us. The only thing I would mention is that the first two or three compressions would not be that fast, so while one had to be careful to step away, the main care was to not fall back after the first step backwards, because by then the engine would be already running.

My own experience, which I don't claim is the best way (and which wouldn't work well with floatplanes :-)) is that I stand to the left of the prop (facing the plane) with the prop at about 11 o'clock.  Right arm outstretched, hand near the end of the prop, you move your whole body briskly to the left.  That way you're always clear of the prop disk.


text:
What is the process for a TFR based on law enforcement activity?
Regarding the recently announced TFR for the Ferguson, MO area, what is the process by which such a TFR is granted?
The official reason given is "TO PROVIDE A SAFE ENVIRONMENT FOR LAW ENFORCEMENT ACTIVITIES", however, there has been significant commentary on social media arguing that this may be a politically-motivated act to prevent the media from scrutinizing the actions of law enforcement (for instance here).
As far as I know, there is no (public) information regarding threats to airborne vehicles, and despite some initial reports most of the protests in the area have been peaceful.  I'd like to assume that this TFR was created for legitimate, non-political reasons.
Is it common for the FAA to grant a TFR like this to local police?  What is the process by which this is done?  What criteria are considered by the FAA in determining whether such a TFR is warranted?

For the sake of posterity, the full text of the TFR NOTAM is:
!FDC 4/0271 ZKC MO..
AIRSPACE FERGUSON, MO..
TEMPORARY FLIGHT RESTRICTIONS. WITHIN AN AREA DEFINED AS 3 NM RADIUS OF
384428N0901812W (ST LOUIS VORTAC STL129011.0) SFC-3000FT TO PROVIDE A SAFE
ENVIRONMENT FOR LAW ENFORCEMENT ACTIVITIES. PURSUANT TO 14 CFR SECTION
91.137(A)(1) TEMPORARY FLIGHT RESTRICTIONS (TFR) ARE IN EFFECT. ONLY AIRCRAFT
PERMITTED UNDER THE PROVISIONS OF 14 CFR 91.137(B) ARE ALLOWED TO OPERATE IN
THIS 91.137(A)(1) TFR. THESE AIRCRAFT MUST BE OPERATING UNDER THE DIRECTION OF
THE UNIFIED COMMAND AT THE ST. LOUIS EMERGENCY OPERATIONS CENTER AND MUST
CONTACT THE MID-AMERICA AIR OPERATIONS BRANCH AT TELEPHONE 636-825-9341 OR
636-825-9342 FOR PERMISSION TO ENTER THE TFR PRIOR TO DEPARTURE. ST.LOUIS
GATEWAY TRACON, TELEPHONE 314-890-1017, IS THE FAA COORDINATION FACILITY.
MEDIA CONCERNS REGARDING THIS TFR SHOULD BE DIRECTED TO FAA PUBLIC AFFAIRS
THROUGH THE WASHINGTON OPERATIONS CENTER AT 202-267-3333. 1411250415-1411251015

Hi chouchand, I've removed two sentences from your question in the hope of keeping focus on the question rather than the event.  Feel free to modify or revert if you feel that those sentences are necessary to understanding the question.

@SteveV. thanks, great edit.

TFRs around "volatile situations" on the ground aren't a daily occurrence, but they do happen fairly frequently.
The concerns are usually twofold: Events on the ground posing a hazard to aircraft (people shining lasers at aircraft, firing guns at them, etc.), and aircraft posing hazards to each other (News 2, News 4, News 11, and Police helicopters all wanting to cover the same area at the same time and smacking into each other).
There are procedures in effect for agencies to request TFRs, and for the FAA to review them. These are covered in detail in FAA JO 7210.3Y, Chapter 19. Particularly relevant to this situation is 19-2-5 (e): "TFRs issued for law enforcement activities require approval from the ATO Director of System Operations Security (or designee)." - a relatively high level of approval intended to safeguard against the use of TFRs to, for example, keep the media away.

In the case of Ferguson the regulatory basis for the TFR is cited as FAR 91.137(a)(1) (to protect persons and property on the surface or in the air from a hazard associated with an incident on the surface), so presumably their major concern is laser illumination or weapons fire from the ground, and the potential for such incidents to cause a surprised pilot to crash into a building or crowd of people.

I suspect you've hit the nail with mentioning news helicopters. They probably don't want to have twenty of them in the area in case something happens and they need a police or emergency one get there.

It is quite common for TFRs to be issued over natural disasters, like hurricanes and tornadoes. In many cases they specifically state that they are issued to keep pilots who are not part of the relief operation from clogging the airspace. They are less frequent for incidents like this, but I have seen them before. There have been reports on Twitter and the media that shots have been fired at police helicopters, so that could be the motivation in this case to restrict the airspace.
The TFR itself says that the reason is “TO PROVIDE A SAFE ENVIRONMENT FOR LAW ENFORCEMENT ACTIVITIES” and I have no reason to doubt it.
What is a bit puzzling about the TFR in this case is that it is in Class B airspace where controllers could presumably keep everyone away anyway. It also covers part of the St Louis airport. It would be interesting to know how that plays out. I guess this just makes it explicit that it could be dangerous.

Is it still B all the way to the ground (I wanted to check on skyvector, but it's boycotting me right now)? It's not that close to the (international) airport.

Its pretty close to the airport, the TFR is inside the Class B surface volume.


text:
Are jet simulators a commercial business or are they owned by airlines?
Are simulators owned by airline operators or are they property of private companies who are specialised in this business and offer the services on an outsourcing basis to airlines that require it?
I'm talking about proper simulators like the Airbus or Boeing ones and not PC based ones.

Both!
Some airlines operate simulator facilities. For example, United has a training facility in Denver with a number of simulators. In turn, they rent out training services to others who will pay to use the facilities (more info on their offerings here). They can also provide trainers, cabin crew training, ground school training, and other such services. Many airlines have similar programs. They've even auctioned off simulator time to customers willing to bid frequent flyer miles for the privilege.
When an airline doesn't own and operate the necessary equipment itself (even an airline that runs its own simulators may not have a suitable quantity for every aircraft in its fleet), it can contract for such services from someone else. That someone else might be another airline, a company that provides simulators (example) and may offer other training services like instructors, or a provider affiliated with the manufacturer (example).

Large facilitators of simulators are CAE and Flight Safety International

I work for an airline with a homogeneous fleet (737), and they did invest in their own simulators. Some of these are certified replica of actual tail numbers, and spares for the real aircraft are used with the sim.

In addition to the good information in @ZachLipton's answer, many aviation training companies offer simulator training for aircraft types other than those that most airlines fly, for example the Kodiak, Citation, or King Air.
This type of training is often used for training individuals flying their own advanced aircraft or for professional pilots. Pilots flying for corporate or charter outfits will typically do initial and recurrent training with such aviation training facilities.
Examples of such training facilities would be the Spokane Turbine Center, Flight Safety International, SIMCOM Aviation Training, and many others.
In addition to these companies that specialize in aviation training for advanced aircraft, many flight schools that specialize in primary flight training also include training in high level FTDs—which have also been used by some airlines, at least in the past. Examples of such schools would be MSU and UND, among many others.

Yes there are private companies that operate Full Flight Simulators and sell simulator time to users, either airlines or self funded students. Pan Am is one example.
Traditional airlines used to acquire their own simulators. However for low cost airlines, pilot training is not core business and a simulator is not an automatic purchase.
There are more and more self funded students appearing in the aviation industry. Pilot training is becoming an issue: there are over 500 new aircraft per year delivered of both B737 and A320. More than 1 per day of each type! Each additional aircraft delivered to an airline (so not a replacement) requires 4 - 5 flight crews. 10 new pilots. 5 captains and 5 F/Os, all trained in a simulator for captaincy and type rating.
To recap: A Level D full flight simulator for an airliner is a 10-20 million dollar device. And there are private companies that arrange bank financing and then sell hours to airlines and self funded students, either with or without instructor services.

As mentioned earlier, most of the major airlines own their own full motion simulators.  In addition, major flight schools also have full motion sims as well as companies like Flight Safety International.
The OEMs also maintain full motion sims and can lease sim time to smaller carriers or flight crews attempting to become type rated in an aircraft which they sell.


text:
Why are landing fees at large airports based on weight?
KORD airport for instance charges domestic vs international arrivals differently.
I could see that this may have something to do with imports/taxes/tariffs etc, but why are the landing fees measured in \$ per 1,000lbs?

comparative damage to the runway and taxi routes

because the costs to handle passengers from a B777 are higher than the costs of handling a C152? Or because the profits on a B777 flights are higher than a C152 flight, so more can be charged.

Because a widebody jetliner takes up a lot bigger chunk of airport than an ultralight does?

From Anne Graham's book, Managing Airports:

This charging mechanism uses the ‘ability to pay’ principles, since
  airlines using larger aircraft are in a better position to pay higher
  charges. Some costs such as runway wear and tear do increase with
  weight and also larger aircraft require vortex separations, which can
  reduce the number of aircraft movements during a certain period.
  Overall, however, there is not a strong relationship between aircraft
  weight and airfield cost. A flat rate landing charge for all aircraft
  types may be more appropriate, particularly at congested airports.
  This is because the cost of occupying the congested runway is movement
  related and independent of aircraft size. Each aircraft movement will
  consume the same resource.

I disagree with Ms. Graham's assertion that the costs are independent of aircraft size.  You can fit a lot more smaller aircraft into a line on a taxiway, and it's been my experience that smaller aircraft use less runway / take off quicker.  Would OSHKOSH function if all aircraft were 777 sized?

@DanPichelman The quote specifically addreses that when it says "Some costs such as runway wear and tear do increase with weight and also larger aircraft require vortex separations, which can reduce the number of aircraft movements during a certain period." and then goes on to say that there is not a strong relationship so I don't know what you are disagreeing with.  Also wake turbulence avoidance procedures call for a specific time between aircraft of a similar size, so a similar number of operations will take place.  Oshkosh is special and operating on a waiver.

Bonus question: does anybody know WHEN is the weight measured?  Is fuel part of this measurement or just airframe?

It is not measured. Most airports have a weight-related landing charge based on maximum takeoff weight (MTOW) which is fixed and constant per aircraft model. It is a design limitation of the aircraft. It therefore does not depend on the actual load the aircraft is carrying (and by load I mean cargo, passengers, baggage, fuel, etc.)

@molgar: That is not entirely true. Each aircraft model will be certified with a certain MTOW, however over time the production process will be refined to eliminate structural weight, plus the calculations will be tweaked based on actual performance, to enhance the MTOW. It is also possible for an operator to certify the aircraft with a lower MTOW than the manufacturers recommendation (such as if they only perform domestic flights, and choose to close off some of the fuel tanks. It is therefore more accurate to say a particular aircraft is certified for a MTOW at the point of registration.

The weight-bearing capability for runways (especially in the touchdown area) and taxiways is determined by the thickness of the runway surface. Heavy jet runways are much more expensive to install, maintain, and replace.
https://commons.wikimedia.org/wiki/File:Runway_cross_section,_Loring_Air_Force_Base.png

The wear and tear of the runway depends very strongly on the weight of aircrafts, so I expect it to be at least taken into account. By discussing the physics of this below, I will show that the effect is stronger than intuition may commonly suggest.
It is certainly not the only factor to determine the fee, and may even be a minor one in most cases, but it could be of practical importance for more uncommon cases, (like when a runway is already in bad shape in some way).
I'll do a rough estimate of the incremental damage caused by a plane, on average, in comparison to planes of different weight.
The wear of the runway and taxiways depend on the load per area that is caused by the wheels. Take note that heavier planes use larger wheels to reduce that load per area.
Regarding the basic physics, the load per area is comparable to the axle load of cars. For the street wear caused by a car, this article [1] states that it is proportional to the axle load raised to the fourth power.
That means a double load per area causes 16 times the wear, or four times load per area 256 times the wear.
Even without an estimate of how much reduction of the load per area is provided by using larger wheels, I think that means that, for all practical purposes, the wear is caused only by the "large" airplanes.
[1]: "Die Kosten der Verkehrsinfrastruktur" Page 12 (German, PDF) refering to work of the american State Highway and Transportation Officials (AASHO) during the 1950ies, see Wikipedia (German) on axle load.


text:
Black boxes: great for crashes on land. How about additional "floaties" for water crashes (low-tech, light, cheap)
Without getting into the mess of redesigning existing Flight Data Recorders, I have a simple proposal that I think would help in deep water crashes.
I propose that several floating cushion sets be distributed around the plane (tail section, along fuselage, etc.). These FDR floaties would be about the size of a seat cushion, but they'd be wrapped in a water soluble cover. When a plane crashes into the water, if the plane breaks up, then several of the cushions would float to the surface. When the cover dissolves, several folded arms open up making it much bigger exposing a orange-nylon covered mesh with an aluminum sheet embedded. This would be easier than seat cushions to see from satellites and planes, and the aluminum layer would reflect radar and make it easier to find.
This alone would help find water crashes sooner, but if you add a simple USB memory stick in the center, then have data similar to the current FDR's being fed into it, then finding one of the floaties would give quite a bit of information including the final GPS coordinates before the crash.
These devices would be light and cheap. I'd think current planes could be retrofitted very cheaply. The only challenge would be the wiring needed to connect to the main FDR or the nearby data splitter. But just putting a few in the tail section alone would end this madness of having to find FDR's on the seabed to get a better idea of what happened.
Wouldn't this be easy to implement without disturbing current FDR use and development? These would be destroyed in a fiery land crash, but that is not their purpose. This is just so we can find plane crashes in the sea when we don't know precisely where they went down (and to get basic data when the black boxes are too deep to get to immediately). Malaysian flight 370 and Air France Flight 447 would have both been greatly aided if these floaties were in those planes. What do you think?

Also related: http://aviation.stackexchange.com/questions/2153/do-flight-recorders-float-in-water?rq=1

This question "what do you think" invites speculation. This website may not be the best place for discussion of ideas for novel tracking devices.

Yeah, I wondered if this was the case when I was composing the question. I almost want to apologize, but for some people, they would know if such an idea was feasible, technologically and implementation-wise as well. It would not be speculation for them to render their opinion on what may be a unconsidered option that would help.
As to me bringing it up here, if not following the letter of StackExchange's outlook, it is following the spirit of it, as I understand.

The first thing that comes to mind to me is that they'd have to be firmly attached enough to not come off in flight, yet somehow detach easily when in the water. They'd also create additional drag, which - especially to commercial air carriers - translates to money lost (additional fuel).
There would also need to be some connection (wired or wireless) to continuously transmit data to them (I'm assuming that you're proposing several of them).
Finally, the whole point of the FDR's housing is that it's rigid enough to withstand explosions and impact, yet the floatation proposal requires that there be some sort of expanding material that'd inflate upon contact (or presumably submersion - consider weather/rain) with water.
General aviation aircraft use ELTs (Emergency Locator Transmitters), which are designed just to notify of the location of a crash incident. They are required in the U.S. for general aviation and commercial aircraft, but unfortunately not for scheduled flights by an air carriers.
I get the impression that the trend is probably towards having more data transmitted real-time during the flight (think Air France stall/crash) of systems monitoring data and location such that location and recovery of more data-intensive logs (voice recorder) would be more efficient.

These floaties would only need to be deployed when the plane hits the water and breaks up, so they would be inside the fuselage. If the plane doesn't break up, then other transmitters automatically come on. When the plane breaks into pieces, several of these floaties would come out too, just like the seat cushions. And they wouldn't need to be inflated. Enhanced seat cushions tucked under the plane's skin here and there is the idea.

@GregConquest and what if the part of the aircraft they're stored in sinks intact, which happens often enough? Wouldn't work... The only way to ensure separation is to have them be either on the outside of the aircraft or have some sort of explosive panel just over them and a fully automated highly reliable ejection system that somehow detects any impact with water that's indicative of a crash yet doesn't trigger during a rough or hard landing, or during heavy buffeting due to wind. Not going to happen, way too high risk of accidental discharge.

jwenting, this is low tech and cheap. You put them around the plane. Just a few coming out is better than looking for blue seat cushions and miscellaneous suitcases. This is only useful if a plane breaks up upon impact. At the most basic level, having no data storage device on the floaties, you could easily scatter a few hundred around a plane. A half-dozen or so also holding flight data located near likely breakage points would be the enhanced level.

@GregConquest And how would you find these "floaties" in the middle of the Pacific or Indian oceans?

Late to the "party" here, and my opinion won't be a popular one (and probably would not be the same, were it my family lost aboard the missing plane, but: in the grand scheme of things, how many planes (and passengers/crew have been completely lost where we have no idea at all where they crashed) over the lifetime of manned flight, as compared to the number of planes and people total. Cost vs Risk vs Reward. Why should an airline pay for something in real money that the chances are infinitesimal to occur?


text:
Is it legal to give flight instruction in my own airplane without 100 hour inspection if doing it for no charge?
14 CFR §91.409(b) states

...no person may give flight instruction for hire in an aircraft which
  that person provides, unless within the preceding 100 hours of time in
  service the aircraft has received an annual or 100-hour inspection and
  been approved for return to service in accordance with part 43 of this
  chapter or has received an inspection for the issuance of an
  airworthiness certificate in accordance with part 21 of this chapter.

What if I want to give instruction to a friend or family and they are not paying me to do it? Would I be exempt from having to do a 100 hour inspection?

"which that person provides" does that means that if the student provides the plane then it will be exempt from the 100 hour inspection as well?

@ratchetfreak Yes.  That is what that means.

Your best bet is to get a legal interpretation from the FAA.  The intent is to ensure the aircraft is in an airworthy state when the student pilot doesn't know what to look at to verify its condition.

Are they paying for the aircraft? Or are you footing that entire bill too? Fuel is expensive, it would be hard to convince an FAA inspector you are giving out free pilots licenses to friends...

@RonBeyer I think who is paying for the plane is a different question, but perhaps a follow-up to this one.
The scenario I am thinking of is: hypothetically I own an airplane for my private use. I am a CFI and a friend and I are flying to Vegas for the weekend. So we are going somewhere incidentally together and could split expenses, can I give him loggable flight instruction on the trip to Vegas without having to have a 100-hour inspection? So, I've provided the plane,  we are going somewhere together that we both want to go, and I want to let him fly as a student pilot on the journey.

An excerpt from 14 CFR 1.1 might be helpful in clarifying the confusion here: Where it is doubtful that an operation is for “compensation or hire”, the test applied is whether the carriage by air is merely incidental to the person's other business or is, in itself, a major enterprise for profit.

@Canuk if you split expenses, that could be construed as him paying you...

I agree with acpilot that if you're merely renting the plane to this pilot then no 100hr is required. Rentals are not "for hire" operations.
I do that with a Cherokee that I rent out. I don’t provide the CFI and I do not fly it for others. Several students have taken their practical tests in the plane and I made sure they could explain the FAR to the DPE and they had no problems.
As several commenters have stated, it gets tricky when you own the plane and provide instruction. The FAA considers lots of things as compensation that you and I would not. Goodwill, logging flight time, paying for gas, or even paying for lunch or a beer.
I found a clarification letter from the FAA at Aviation Banter.
http://www.aviationbanter.com/showthread.php?t=11563&page=2
May 3, 1984
Mr. Perry Rackers
Jefferson City Flying Service
Dear Mr. Rackers
This is in reply to your request of May 1, 1984, that we render an opinion regarding the applicability
of the 100-hour inspections requirement of Section 91.169(b) of the Federal Aviation Regulations to rental aircraft.
Section 91.169(b) of the Federal Aviation Regulations provides that, except as noted in Section 91.169(c),
a person may not operate an aircraft carrying any person, other than a crewmember, for hire, and may
not give flight instruction for hire in an aircraft which that person provides unless, within the previous 100
hours of time in service, the aircraft has received either an annual or a 100-hour inspection.
If a person merely leases or rents an aircraft to another person and does not provide the pilot, that
aircraft is not required by Section 91.169(b) of the Federal Aviation Regulations to have a 100-hour i
nspection. As noted above, the 100-hour inspection is required only when the aircraft is carrying a
person for hire, or when a person is providing flight instruction for hire, in their own aircraft.
If there are any questions, please advise us.
Sincerely,
/s/
Joseph T. Brennan
Associate Regional Counsel
One correction to acpilot about the 100 hours. Once you start renting for hire, you need to do an inspection every 100 hours. The clock doesn’t restart every time you do the inspection or annual. You are allowed a little leeway to ferry the plane to the inspector, but you don’t reset the clock.
The 100-hour limitation may be exceeded by not more than 10 hours while en route to reach a place where the inspection can be done. The excess time used to reach a place where the inspection can be done must be included in computing the next 100 hours of time in service.

Thanks for digging this up, however, it doesn't define "for hire." The scenario is: I want to, for free (no money given to me, but my cousin pay for the costs associated with the airplane (fuel, maintenance reserve, etc)), give a flight lesson in my own airplane to a cousin (for example) and allow him to log the time toward his Private Pilot Certificate. So the Flight Instruction is not "for hire," as would be normally defined. ("To engage the services of (a person) for a fee") I am providing the CFI (myself) and the Airplane (I own it), but am not charging for the Flight Instruction.

I didn’t define 'for hire' because the FAA hasn’t clearly defined it. I have a post at http://www.touringmachine.com/Articles/regulations/478/ that references several letters of interpretation involving 'sharing expenses' that, while not on point, explore the FAAs thoughts about compensation.

Canuk the CFI not logging the time is the only way to give the instruction "not for hire", however I've found that different FSDO come up with different interpretations sometimes. Get it in writing from your local before you give "not for hire" instruction...also instructor insurance isn't a bad idea in case your friend does something stupid.

The CFI can log the time. Flight hours can be considered compensation if the flight hours are available at a reduced rate or something. If the CFI is paying for all costs and receiving nothing in return they are not being compensated.

If you are providing the training and the aircraft truly for free and receiving nothing in return, it is not for hire and no 100 hour inspection is required.
Keep in mind that the FAA considers things besides money to be compensation. If there is some kind of quid pro quo relationship with your students you can expect the FAA to consider this as a for hire operation, which would make it illegal.
A lot of people you'll talk to will parrot the phrase "flight time is compensation." By this logic, it is illegal for a private pilot to log time while carrying passengers. Obviously this isn't the case- flight time is something that can be considered compensation if the pilot does not pay for the full cost of the flight.

If the CFI owns the plane then he must have a 100hr done even if the instruction is "free" in the sense that the student pays nothing.  However, in the eyes of the FAA, flight time is compensation so the flight will not have been "free" in the context in which you are posing the question.  You will have been compensated in dual-given flight hours even if you bear 100% of the cost.  If you own the plane and have had an annual performed in the last 100hrs, you're good to go.  Now, if you're merely renting the plane to this pilot then no 100hr is required.  Rentals are not "for hire" operations.  You can fly along with your friend but you cannot legally log hours.
If the student owns the plane and hires you then no 100hr is required (the way I read it).
Link to the FAA regulation (as of 13 July 2016): http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFar.nsf/FARSBySectLookup/91.409
Link to another helpful article: https://blog.globalair.com/post/When-is-the-100-Hour-Inspection-for-Aircraft-Used-for-Rental-and-Flight-Instruction.aspx

First follow up: if less than 100 hours have passed since annual, then it would work, you're saying? Second follow up: if my friend is renting it, then I believe I could log hours. That is how many clubs work: the student pilot rents the plane and the CFI logs dual-given and gets paid for their time. (If I'm reading the regulation right this makes sense: the person giving instruction hasn't provided  the plane, the student has)

First: for the purposes of this operation a 100hr and annual are the same thing.  An annual within 100hrs satisfies the requirement.  Second: you cannot legally log dual-given in an airplane you provide that has logged >100hr since the last ann/100hr.  The club model does not have the CFI (you) providing the plane (your plane).  You are providing the plane in this case no matter how you slice it.  I would be shocked if the feds arrived at a different conclusion.

So just to be clear, you think that I could log dual-given as long as it had been less than 100 hours since the annual? Additionally, you're saying the for hire clause is always in play because even if money doesn't change hands, you've received benefit as being able to log dual-given hours?  I guess I'm getting tripped up on what is the definition of for hire in the FAR? You're getting benefit as being able to log dual-given but I would think for hire means money changes hands.

This one might be worth a letter directly to the FAA asking them to clarify the topic. Hear it from the horse's mouth, as it were.

@Canuk "for hire" generally just means you are offering something in exchange for compensation, but compensation is is not limited to money. The FAA has considered logged flight time as compensation. I second the idea that if you want anything more concrete you ask the FAA.

Yes, you can log dual-given if the plane has logged less than 100hrs since annual. Regarding compensation, the reg does not say "money." It says "for hire." What if you exchange your CFI services for corn futures or bags of exotic spices? That's compensation, and so are flight hours. I'll go against the grain here and say DON'T ask for clarification from chief councel. These clarifications can have unintended consequences. I don't see an upside for a clarification request given that the reg (and its intent) is already pretty clear.

@acpilot I'm curious what the "unintended consequences" might be

When the FSDO answers your reg question it's not really a hard and fast decision.  It's an answer to your question according to some guy at a field office (think of it as "yeah...I'd pursue a violation for that.").  When chief councel issues an opinion...that's the way it is.  Clarifying a simple reg like this has little up-side.  Posing the question to chief councel opens the door for a restrictive answer.  If left unclarified, a FSDO can excercise discretion in application of this reg.  When clarified by chief councel the FSDO's hands are tied and enforcement action is a near certainty.

Dual flight instruction can be given without the CFI logging the time. This alone leaves open the possibility of flight instruction that is not for hire. The FAA makes a distinction between flight instruction that is for hire, and flight instruction that is not for hire; this is why the for hire clause is added to the reg.

@JonathanWalters if the CFI doesn't log the hours, can the student still log those hours as experience towards a certificate?

Being "compensated" by logging time when you're paying for all the expenses is nonsense. By that logic, a private pilot can't log time anytime they have passengers!


text:
How can the concept of "intercept geometry" be explained in simple terms?
Can someone expand a bit for the layman with emphasis on Intercept geometry the following image depicting the envelope of a SA-2 missile against a moving Target.

Intercept geometry is the bubble of space in which the SA-2 missile can be effective against a specific aircraft. Crucial here is the detection range of the radar system guiding the missile, given here as 40 nm.
Depending on the speed of the target, the missile has only a limited time to ascend to the altitude of the target and to close in. The combination of speed and altitude of the SR-71 (1854 kt), moving at more than 30 nm per minute, will leave only a very small intercept geometry, and it has to overfly the missile launcher almost directly to be vulnerable. Once the target flies above 90.000 ft and Mach 3.5 (or employs effective countermeasures), it will be unassailable by the SA-2.
Contrast that with the fate of an aircraft flying only Mach 1 at 60.000 ft (which equals 573.6 kt): Moving at less than 10 nm per minute, its course can go past the launcher with a 25 nm offset, but it will still be in range of the Fan Song - SA-2 combination.


text:
What considerations drove the choice of the Solar Impulse engines configuration?
Solar Impulse makes me think of a motorglider due to its aspect ratio.
The Upward cranked tips wing configuration along with the engines configuration particularly draw my attention:

The 2 innermost engines are almost close to the thrust line
The 2 outermost ones are right where the wing crank.

Question:
Was the engines configuration mainly adopted out of structural optimization sake or do other relevant considerations (such as ease of flight control in case of asymetric engine failure etc.) come into play?

It reduces moments in the wing spar. Major points of axial drag in this design are the wings and the fuselage. You'll see that two engines are close to the fuselage, and the other two are midway along the wing.
By placing two engines further out, they are also located where the lift is generated, reducing the necessary strength of the wing spar.

Both of the above are especially important since it's such a large wing.

There might also be some possibility of using asymmetrical thrust to ease crosswind landings with this configuration from the yaw from the outboard engines.

The two inner engines are in the expected position. Keeping the engines as close to the fuselage as possible has two main benefits:

Less adverse yaw in the case of an engine failure
Less stress on the wing on the ground and during landing from the engine weight

The outboard engines are certainly unconventional. Most aircraft with four engines, expecially propeller-driven aircraft, tend to keep them all as close to the fuselage as possible. Some considerations that may have affected this decision:

Less stress on the wing in flight. The weight of the engine is offset by the lift produced on the wing. Large aircraft generally use the fuel in the wing tanks last for this reason.
Putting the engine at a point that is already structurally important can save some weight by combining the structure. In this case:

The wing angles upwards here, so there would probably be a bit more structure to support this bend
The outboard landing gear is also here

This could also mean less weight in wiring to hook up the engines to power

It is interesting to note the design of the Solar Impulse 2. There seems to be a different compromise between the two cases above, with both engines on each wing placed together roughly in the middle of the span. This seems to favor the case of saving structural weight.


text:
G1000: Is it possible to balance the radio volume for pilots and passengers?
I regularly fly a DA42 4-seater equipped with a G1000 system. Sometimes my passengers in the rear seats complain that the radio volume is too loud for them but it is OK for me (intercom is just right for everybody). Reason probably is that I am using a very different headset than them which I assume has a higher impedance and thus requires more electrical power for the same audio volume. If I turned down the COM volume it would be too quiet for me.
I wonder if there is any trick to adjusting/balancing the volume in such a setup.

Do the headsets have their own volume controls, or are you limited to controlling the volume via the G1000 itself?

Nowadays it's hard to find a headset without its own volume control,  but you are right that was my first guess, too. Thomas, if your passenger headsets offers this possibility you maybe advice your passengers at the beginning of the flight that they can set their volume independently on their headsets. But maybe they are simply right and we as pilots, concentrated on the radio, don't feel like it is too loud, while someone enjoying the landscape feels disturbed by ATC. Maybe it's only some kind of a psychological problem. Try a detailed PAX briefing and also mention the importance of ATC.

I tell my passengers if they don't like what's coming over the radio, feel free to un-plug the headset.

You can't control the radio volume separately for the passengers but you can control the intercom volume separately. If the passengers' headsets have individual volume controls you can ask them to turn down the volume and you can then compensate by turning up the passenger volume for the intercom. That way the overall intercom volume will be about the same but the radio/ATC volume will be lowered.
If all else fails, you could turn off the radio completely for your passengers :-)

Intercom Volume Control is the rotary knob on the bottom of the Audio Panel. The small knob controls the pilot ICS volume, while the large knob controls the copilot/passenger ICS volume. Just be sure to be in VOL mode (VOL in the lower left to be illuminated).
Volume for Coms/Navs are controlled by the COM radio volume control located on both the PFD and MFD, but (as far as I know) you cannot set that volume independently for pilot/copilot/passenger.


text:
Can a convergent nozzle be used for a supersonic jet engine?
Can a supersonic fighter incorporate a convergent nozzle or does it always have to be a convergent-divergent nozzle?

This is helpful.

Short answer: Yes.
A convergent nozzle will not allow supersonic exit speeds of the combustion gasses, but due to their high temperature their speed of sound is considerably higher than that of the surrounding air. For example, at 700°C the speed of sound in air is 625 m/s. Since thrust is mainly determined by the difference in entry and exit speeds of the air flowing through an engine, a higher speed than flight speed is required for positive thrust. Low supersonic flight speeds are entirely possible with a convergent nozzle.
Long Answer: Better not.
Once the aircraft crosses the sound barrier, drag increases only slowly since the drag coefficient actually drops. If the design is meant to fly supersonically, which implicates a lot of design adaptions, it makes sense to go for higher supersonic speed; however, this requires both an adjustable intake and an adjustable convergent-divergent nozzle. Both increase efficiency, dramatically so at higher Mach numbers. The simple pitot intake of the F-16 is just good enough for Mach 1.6 (above that its efficiency becomes outright awful), but still its F-110 engine has a con-di nozzle, allowing top speeds of Mach 2.0.
With an adaptable convergent nozzle the exhaust gasses can be accelerated up to their speed of sound, but more only after leaving the engine, given a suitable pressure gradient. However, this does not help to increase thrust as it happens behind the airplane.
A narrowing of the flow path will accelerate subsonic flow, but only to the speed of sound. Only a divergent flow path will then accelerate the flow further to supersonic speed. A convergent nozzle will be shorter and lighter, but means throwing away a good deal of useable energy with the hot exhaust gasses. In most cases it would be more efficient to convert the pressure energy of the exhaust gasses fully into more thrust by accelerating them to supersonic speed.
Convergent nozzles only make sense if the aircraft is designed only for short, limited supersonic dashes, but will spend almost all flying time at subsonic speeds. Examples are the Northrop F-5E or the Panavia Tornado.

I’ve actually been trying to figure this out (on and off) for weeks now. So, in a C-D nozzle, when the sonic flow reaches the divergent part, how is it able to accelerate? The flow is moving at the speed of the molecules themselves, so how could they possibly accelerate by any sort of pressure gradient? If this isn’t a clear question, I can try to clarify more.

@Wyatt Did you ever read this?

I actually have read that answer a few times over because it didn’t click immediately. I’m currently not really seeing how it helps to answer my C-D nozzle question. Perhaps this question is more suited for somewhere like Space.SE?

@Wyatt Every time a flow is accelerated it trades pressure for speed. I honestly cannot fathom why that would be hard to understand.

That does make total sense when the air accelerates to still subsonic speeds. Imagine you have a vacuum chamber connected to a reservoir of extremely high pressure air with a valve. When you open this valve, the air will accelerate towards the vacuum side. When this air reaches ~M = 1, it is no longer able to accelerate because the air is moving at the speed of the molecules themselves. I'm asking what the divergence of a nozzle does to allow the flow to accelerate further.

Hmm. I've actually thought better of this question. I think the only question I have now is if air (or any gas) can accelerate faster than the speed of its molecules. And, I was wrong in the last part of my comment, because the speed of the air molecules themselves is actually M = 1.45, not ~M = 1. Thanks for your time.

Can a supersonic fighter incorporate a convergent nozzle?

It actually exists.
The Panavia Tornado has a maximum airspeed of Mach 2.2 at 9000m altitude and has variable geometry convergent nozzles (although they work only for the afterburner, while the turbine's nozzle is convergent-divergent):

Photo from Airliners.net

That is a convergent-divergent-convergent nozzle

@DeltaLima could you find me a schematic? if I'm wrong I'll delete the answer, but sincerely I would not understand its usefulness.

I am looking for some schematic, but I can't find a decent one. If you look into the engine you can see the turbine exit nozzle diverging, then the afterburner and then it converges again. The geometry of that nozzle is controllable. I assume that final small convergence is to prevent over-expansion and to create oblique shock waves.

@DeltaLima then I would say that I am "only" partially wrong, as the A/B will only see the last convergent part.

I didn't mean to say you were wrong or that you should delete your answer. It was just an observation when I looked into the business end of this beast.

@DeltaLima  no problem at all, I took no issue with your comment, but to reflect it I updated my answer.

The Mach 2.2 speed of the Tornado is a data sheet value which can be flown only in the best of circumstances. More realistic is the experience that Tornados need to keep one engine on afterburner just to keep up with tankers at mid altitudes. The RB-199 is optimized for low level and nothing more.

@PeterKämpf I spoke with a Tornado pilot and he reported that it can easily go supersonic. He's not allowed to go in the details, but supersonic for sure.

@Federico: Yes, of course it can. But supersonic is everything above Mach 1. Mach 2.2 is quite a bit faster than Mach 1. Start to put external tanks and some ordnance on those swing wings, and it will be hard to follow an airliner with dry thrust only.

@DeltaLima: I think Federico is right, this is a convergent nozzle. It needs to be adjustable for the afterburner, but adjustability doesn't make it a con-di nozzle. The Tornado was designed to be as short as possible, and the RB-199 was selected because it supported this goal. And since it was designed for near-sonic low-level flying, this made sense. The British ADV version is a perversion of the initial design and needed a reworked engine, the Mk 104.

Pilot here, as well as an engineer who made it most of the way to an Airframe and Powerplant technician license at one point in life and worked a lot on nozzles including at the jet Propulsion Laboratory (very fun).
If you want to understand nozzle shapes a fun introduction is to read about the shape that inspired modern rocket (and some jet) nozzles, the de Laval nozzle.
Let's keep in mind that what we want (ignoring for the moment optimization of air volume to feed combustion) is HIGH SPEED in the OUTGOING JET EXHAUST.
In a SUBSONIC flow the air remains compressible and a converging subsonic nozzle increases velocity while pressure and temperature decrease. (There are actually about a dozen subcases here that I'm sure others will yell at me on, so consider that P and T decrease to be an inexcusable simplification on my part to avoid another page of explanation.) So for a slower exhaust final speed, a converging nozzle makes perfect sense.
The ideal situation for SUPERSONIC flow is converging-diverging (see the de Laval again) because, for isentropic flow, the converging portion builds energy up to the point of the sonic speed. This sonic point is then put right at the "throat" where converging becomes diverging and the diverging part turns heat and pressure into more speed up to fairly high values of supersonic. Note that a modern jet engine is fairly complicated in air flow and the "throat" may be deep inside the engine and not look like a rocket engine throat at all.

For these reasons we would expect to see converging final nozzles on subsonic aircraft like airliners and some fighter jets, while supersonic jets usually have diverging nozzles. If you see a converging nozzle on a supersonic-capable fighter look at it carefully to see if it is articulated for changing shape in flight - which is pretty darn common.
Having said all that, aerospace designers are very clever and injecting fuel (afterburners) can cause non-isoentropic flow if designed for it. So I won't go out and say there aren't any converging nozzle supersonic nozzle jets, but at least now you know why they would be the exception and not the rule :)
Please feel free to post questions if I skipped something that should be addressed...

There is a school of thought that contends that the convergent nozzle is an inefficient version of the convergent -divergent nozzle.  The jet velocity will rise to a maxima at the throat of the nozzle (smallest area) and then exit the nozzle and expand again against atmospheric pressure.
You will find that a fixed convergent nozzle would not be used because a fixed nozzle of any sort will be less efficient than a variable geometry nozzle.  The ability to tune the nozzle for local atmospheric pressure, airspeed, and engine throttle setting is key to efficient generation of thrust.
Could it be used?  Yes
However the circumstances where it is the best option are probably so limited as to make it never the right choice.

Never say never … if the design calls for short supersonic sprints only, the mass and size savings of a convergent nozzle might be preferable and pay for the extra fuel that needs to be carried due to the lower efficiency.

The point is well made but sprint interceptor type aircraft of the type you suggest (like the English Electric Lightning or Lockheed Starfighter) typically use variable geometry nozzles so that they can use afterburners.  Maybe some early jets used this design however.  Perhaps the Gloster Meteor or Messerschmidt ME 262?

Why do you equate variable geometry with a con-di nozzle? A convergent nozzle needs also to be variable if used behind an afterburner. And both the Lightning and the F-104 were designed to spend a considerable part of their airborne life at supersonic speed, so they are bad examples. Better look at the F-5 or the Tornado.

I don't think i'm equating the two in my answer.  The question seemed to be asking about a fixed con-di nozzle.  All the jet pipes I have experience with or have done design analysis of used a variable geometry to better adapt to different conditions.  It therefore seemed appropriate to expose the questioner to the rationale behind using a more complex design.  Any answer without showing how it would work in a specific example is a bit lacking but I don't have the time to do a worked example at the moment.  Perhaps I will revisit this later.

Even the Me-262 had a variable con-di nozzle - a fixed one would not make sense at all, because it could only work at one thrust setting, speed and altitude. I am not aware of any jet engine with a fixed con-di nozzle.

For a supersonic plane, a convergent nozzle with decelerate the flow. This is required only at the compressor, where flow velocity should be slower, but after that to get more thrust, the flow needs to be accelerated .
Now, one can argue that if a supersonic flow enters a converging nozzle, it would accelerate so subsonic speed (at about Mach 1 ) after which the convergent nozzle would accelerate the flow. But this would never achieve any Mach above 1 for the flow, because as soon as the flow tops Mach 1, it would be decelerated by the convergent part of the nozzle once again. So, the maximum exit Mach one could expect to get off a convergent nozzle is 1 (whether the input is supersonic or subsonic).
Now, for supersonic flight, it is necessary that the exit mach number is greater than 1. In such cases, one has to use the Convergent Divergent Nozzle (C-D Nozzle).
If some one wishes to build an engine which can accelerate a plane to supersonic velocities with subsonic (or transonic) exhaust velocity, then for sure one can use convergent nozzles only. But, there is a catch :
The thrust equation is:

Here for an engine in free stream Pe is approximately equal to P0. So for positive thrust, the terms containing Ve and V0 should give a positive answer. So to create an engine where Ve < V0 (because V0 is the free stream velocity which is supersonic, and the Ve is exit velocity which is assumed to be transonic in this case), one has to vent out a lot of Me at a high rate. This will mean that it is technically possible to create an engine which gives supersonic flight at transonic exhaust velocity, but such a configuration is going to cost you a lot of fuel.
Now, if you consider that the exit temperature is very high, lets say around 1200k, and lets say input temperature of free steam is about 300k, that means the exhaust velocity can be about Twice the Mach 1 velocity of air at 300k (without being supersonic), i.e. about 630 mps. If you input mach is about 1.2, that tranlate to input freestream velocity of 380 mps (apx). This means your exit has accelerated to 630 mps from 380 mps, just by the means of a convergent nozzle. But this still would require a lot of fuel to be burnt (because the thrust required usually for flying a plane a few thousand kilograms in weight is very high).
Hence, its possible, but at the expense of the efficiency of the aircraft

Hint: Speed of sound changes with temperature. Even when the exhaust flow is subsonic, it will me much faster than the intake flow, even with a convergent nozzle. Low, short supersonic flight is actually quite efficient that way, because less of a nozzle needs to be carried around.

Editted the answer. Thanks

Acceleration or deceleration in the nozzle have nothing to do with flight speed. A convergent nozzle will accelerate subsonic flow up to sonic speed (1st sentence). What decelerates flow near the compressor is actually called a diffusor.

A diffuser is a nozzle


text:
Why evacuate wing at the front side after water landing?
When looking at the following Transavia B737 safety cards, I noticed that after a water landing, passengers who evacuate via the overwing exits should leave the wing at the front, while during evacuations on land they should leave via the rear side of the wing.

It is clear for me why normal evacuations need to go via the rear side of the wing, but why does a water evacuation need to happen via the front side?

Does it have to do with the center of gravity position, in an attempt to keep it is far forward as possible so that the tail doesn't sink? Because in the second picture the rear exits are prohibited from use too.
(images cut pictures at this source)

Its an A320 but take a look at how it is sitting in the water. You don't want to open the rear doors because chances are it is pretty heavy in the back and will be tail-down.

I don't think it's necessarily heavy in the back. On US Air 1549 they didn't have time to hit the "ditch switch" which closes the outflow valve. That allowed water to enter at the back of the plane. I'm pretty sure if the crew had time for the entire procedure it will float level

I'm very impressed that you read the safety card so closely.  Not many people do.

@PJNoes Unlike many people, I fully intend to get out of the aircraft when the flight ends!

@TomMcW The ditch switch would help, but wasn't the cause of it sinking. A panicked passenger cracked the rear door allowing water to rush in. There was also fuselage damage in the rear that caused cargo doors to open and also flood. Considering most ditchings are tail-low and many have rear cargo doors (and cargo), I'd guess that most planes would sit tail-low in the water like that.

@PJNoes Many people on here fly too often to not be concerned with safety ;)

Many people get so bored during flight that they even read the safety card!

@PJNoes: I do!

When I worked as a flight attendant back in the 80's we were told that water landings frequently resulted in very damaged trailing edges to the wings.  Flaps and spoilers will most likely have been deployed prior to landing and the velocity of the water impacting these extended surfaces would tear them up badly, along with the hinges and fairings that support them.  To avoid serious injury to passengers from ragged sheet metal, we were to direct them off the leading edge of the wing.
This wouldn't be a problem with an evacuation on land so in that case we would direct them off the trailing edge.  The upper wing surface makes a good slide and a more natural and controlled drop to the ground.
Edit:  I forgot to mention that additionally, water landings mean passengers would be wearing life vests and life rafts would be deployed.  Jagged sheet metal would make short work of these inflated devices as well.

Can't argue with an answer with insider knowledge. Thanks for coming by to help out!

I always thought that the reason to go off the back of the wing was to avoid getting sucked into an engine.  So I learned something today.

I was thinking on a emergency (land) landing, Passengers would go down the trailing edge to alleviate the distance to the ground.  Not sure if that is really a thing.   If the trailing edge is damaged during a water landing, definitely makes sense to depart off the leading edge.

I'd imagine on a land landing, the leading edge could bring a passenger dangerously close to being ingested, should the engines be rotating..

@MichaelSeifert I find that whole scenario difficult to envision, but decided to ask the experts. So Is there any situation in which engines would not be shut down before emergency evacuation of an aircraft?

You can't put every contingency on a safety card and not included on the safety card but a standard part of evacuation procedures is to check outside the window to make sure it's safe before opening any emergency exit.  Over wing exits are supposed to be opened by the passengers in those rows, but a flight attendant should go in that direction after evacuation starts just in case they can't figure it out and to help out.  Presumably a water landing won't include a running engine after landing.  On the ground, a bit of awareness and common sense would go a long way.

@PJNoes sadly, awareness and common sense are rare commodities (in the US, at least), especially so in the panic immediately following an announcement to evacuate.

@MichaelKjörling In addition to the QF32 situation mentioned in the answer to your question, if the engines were at a high power setting when the crash occurred (like in Asiana 214, where they were unsuccessfully trying to go around,) it would take them quite a while to spin down to non-dangerous levels, especially with the monsters they have on a plane like the 777.

@reirab How long? Not asking for an exact number of seconds, but for someone who isn't all that knowledgable about this, can you give us an idea? Are we talking half a minute, a minute, five minutes, what?

@MichaelKjörling Honestly, I'm not really sure and it would depend on the engine and conditions. It would presumably spin down quite a bit faster in water (and possibly also detach.) From a quick search, I didn't find any videos showing a shutdown from a full power setting, but this one shows a PW2043 (from a 757-300) shutting down from idle after parking. From idle, the first 15-20 seconds are still around the "come close and I will eat you" speed, though it's still spinning fast enough to perform an amputation for around 1.5 minutes or so.

An answer that I once received from a flight attendant (I don't remember the airline or plane model, it was long ago) was that in the event of a water landing passengers would be able to assist each other if they were all together. Thus, passengers leave by the front of the wings in the direction of the other usable exits.
Otherwise, the wings would separate the passengers into groups.

Additionally to @PJnoes answer, I spot that the path to the front of the wing is dangerously close to the (possibly hot) engines. That's less likely an issue when they are immerged into cooling water and may explain that going towards the tail of the aircraft is preferred in case of a landing on the ground.

The main reason for going aft on ground landing is that it is still rather high (B737 does not have wing slides) and the extended flaps will provide a makeshift slide aft. While on water the lack of wing slide means the evacuees need to go towards the forward slide, which is the only available inflated flotation device around.

It all depends on the design of the exits as well as how the aircraft floats in the water prior to succumbing.  It may have been determined that it would be easier for people to deplane at the leading edge of wing it water as opposed to the trailing edge for that reason.

For comparison, here's a picture I've taken of an emergency card onboard an Airbus A319. Slides from the wings are deployed after emergency landings both on land and on water. I guess the slides have to be specially protected against damaged trailing edge of the wings, as discussed above.

(Image source: Own work)